don't blow up if a single download is not trackable.

src/NzbDrone.Core/Download/DownloadTrackingService.cs

@@ -139,6 +139,8 @@ namespace NzbDrone.Core.Download
             {
                 var downloadClientHistory = downloadClient.GetItems().ToList();
                 foreach (var downloadItem in downloadClientHistory)
+                {
+                    try
                     {
                         var trackingId = String.Format("{0}-{1}", downloadClient.Definition.Id, downloadItem.DownloadClientId);
                         TrackedDownload trackedDownload;
@@ -147,7 +149,8 @@ namespace NzbDrone.Core.Download
 
                         if (!oldTrackedDownloads.TryGetValue(trackingId, out trackedDownload))
                         {
-                        trackedDownload = GetTrackedDownload(trackingId, downloadClient.Definition.Id, downloadItem, grabbedHistory);
+                            trackedDownload = GetTrackedDownload(trackingId, downloadClient.Definition.Id, downloadItem,
+                                grabbedHistory);
 
                             if (trackedDownload == null) continue;
 
@@ -159,6 +162,11 @@ namespace NzbDrone.Core.Download
 
                         newTrackedDownloads[trackingId] = trackedDownload;
                     }
+                    catch (Exception e)
+                    {
+                        _logger.ErrorException("An error occured while tracking download." + downloadItem.Title, e);
+                    }
+                }
             }
 
             foreach (var trackedDownload in oldTrackedDownloads.Values.Where(v => !newTrackedDownloads.ContainsKey(v.TrackingId)))